1. Field of the Invention
The present invention relates to a battery charger for charging rechargeable batteries such as nickel-cadmium batteries.
2. Description of the Related Art
In conventional battery chargers, while supplying a constant level of charge current to a battery, a battery voltage or a battery temperature is sampled at every constant interval to detect that a battery has reached a fully charged condition. Japanese Laid-Open Patent Publication (which is A type publication and will be referred to as "OPI publication" hereinafter) Nos. SHO-53-103544 and SHO-54-158641 describe battery chargers that determine whether a battery has reached a fully charged condition based on the battery voltage periodically sampled at every constant interval. OPI Publications Nos. HEI-2-246739 and HEI-6-113475 describe battery chargers that determine full charge of a battery by detecting change in battery temperature while periodically sampling the battery temperature at a constant interval. On the other hand, U.S. Pat. No. 5,410,238 to ISHIZUKA et al proposes changing the level of charge current according to an initial battery temperature. OPI Publication No. HEI-6-6939, which corresponds to co-pending U.S. application Ser. No. 08/77,051, proposes changing the level of charge current depending on the remaining capacity of the battery to be charged.
Generally, when a battery is charged with a small charge current, for example, with 5C where C is a battery charging rate and represents a charge current required for charging a battery in an hour, the battery voltage and battery temperature change only by a small amount per unit of time as shown in FIG 1A. Contrarily, when a battery is charged with a large charge current, for example, with 8C, the battery voltage and temperature change by a large amount per unit of time as shown in FIG. 1B. The greater the difference between a large and a small charge current, the more pronounced the difference between their charging characteristics, i.e., change in voltage and change in temperature.
When the charge current is changed dependant on change in battery temperature or change in battery voltage as detected by sampling, the smaller the applied charge current, the smaller the change in battery voltage and the change in temperature per unit of time, resulting in a long charging time.
When charging is performed with a small current, battery voltage and battery temperature change only slightly during charging. Therefore, if sampling is performed at a relatively short interval as is done in the case of charging with a large current, the full charge may not be detected despite full charge being actually reached, resulting in overcharge of the battery because charging is unduly continued. On the other hand, charging with a large current increases change in battery voltage and change in battery temperature. Therefore, if sampling is performed at a relatively short interval, the full charge can be accurately detected. However, if sampling is performed at the longer interval used for charging with a small current, full charge may be detected after full charge has actually been achieved, again possibly resulting in an overcharge.